What is "back pressure"

Pressure \Pres"sure\ (?; 138), n. [OF., fr. L. pressura, fr. premere. See 4th Press.]

1. The act of pressing, or the condition of being pressed; compression; a squeezing; a crushing; as, a pressure of the hand.

2. A contrasting force or impulse of any kind; as, the pressure of poverty; the pressure of taxes; the pressure of motives on the mind; the pressure of civilization.

   Where the pressure of danger was not felt.
   --Macaulay.

3. Affliction; distress; grievance.

   My people's pressures are grievous.
   --Eikon Basilike.

   In the midst of his great troubles and pressures.
   --Atterbury.

4. Urgency; as, the pressure of business.

5. Impression; stamp; character impressed.

   All saws of books, all forms, all pressures past.
   --Shak.

6. (Mech.) The action of a force against some obstacle or opposing force; a force in the nature of a thrust, distributed over a surface, often estimated with reference to the amount upon a unit's area.

7. Electro-motive force.

   Atmospheric pressure, Center of pressure, etc. See under Atmospheric, Center, etc.

   Back pressure (Steam engine), pressure which resists the motion of the piston, as the pressure of exhaust steam which does not find free outlet.

   Fluid pressure, pressure like that exerted by a fluid. It is a thrust which is normal and equally intense in all directions around a point.
   --Rankine.

   Pressure gauge, a gauge for indicating fluid pressure; a manometer.

Back \Back\, a.

1. Being at the back or in the rear; distant; remote; as, the back door; back settlements.

2. Being in arrear; overdue; as, back rent.

3. Moving or operating backward; as, back action.

   Back blocks, Australian pastoral country which is remote from the seacoast or from a river. [Webster 1913 Suppl.]

   Back charges, charges brought forward after an account has been made up.

   Back filling (Arch.), the mass of materials used in filling up the space between two walls, or between the inner and outer faces of a wall, or upon the haunches of an arch or vault.

   Back pressure. (Steam Engine) See under Pressure.

   Back rest, a guide attached to the slide rest of a lathe, and placed in contact with the work, to steady it in turning.

   Back slang, a kind of slang in which every word is written or pronounced backwards; as, nam for man.

   Back stairs, stairs in the back part of a house; private stairs. Also used adjectively. See Back stairs, Backstairs, and Backstair, in the Vocabulary.

   Back step (Mil.), the retrograde movement of a man or body of men, without changing front.

   Back stream, a current running against the main current of a stream; an eddy.

   To take the back track, to retrace one's steps; to retreat.

Back pressure refers to pressure opposed to the desired flow of gasses in confined places such as a pipe. It is often caused by obstructions or tight bends in a confined space such as an exhaust pipe.

Because of air ressistance, friction between molecules, the term back pressure is misleading as the pressure remains and causes flow in the same direction, but the flow is reduced due to resistance. For example,a stock car exhaust system with a particularly high number of twists, bends, turns and right angles could cause lots of back pressure to the gasses escaping the car's engine therefore reducing the flow of the gasses.